Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190733
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
133 AM MDT Wed Jul 19 2017

.SHORT TERM...Tuesday through Thursday night

The weather will again take a more active turn as some monsoonal
moisture begins to invade the area from the south and west. As a
result, look for an increase in coverage of showers and thunderstorms.
This time, it will be mainly across the southern two thirds of the state,
and especially along and south of a Kaycee to Riverton to Afton
line. We continue to agree with the previous shift that the
coverage of thunderstorms to the south of Wyoming across Colorado
and Utah will rob a lot of the moisture so coverage should not be
huge. Look for another hot day across the state, especially in the
northern zones where there will be more sunshine. The normally
warmer spots like Greybull and Worland could get close to 100
today.

The coverage of storms looks to peak on Thursday with aid of a weak
shortwave moving through the area. As a result, we have increased
coverage of the storms across most of the area with the exception of
far northwestern Wyoming. Again, the best chance of storms will be
across southern portions of the county warning area, roughly south
of a Casper to Afton line. This is the day the thunderstorms have a
chance to be of the wet variety as well although pinpointing the exact
location of the storms is very difficult at this time.
Precipitable water values do climb close to an inch across the
area though so there could be some isolated flooding. With more
cloud cover, temperatures will be a bit cooler than on Wednesday.

Things then dry out for Friday. Much drier air from the southwest
will spread across the state and bring much lower dew points and
precipitable water values. As a result, most of the area looks
dry. We think continuity looks good with keeping any chance of
thunderstorms along the southern border where a bit of monsoonal
moisture may hang on for longer. Temperatures will be on the
upswing once again as well as there will be more sunshine. In
addition, with the very dry air and possibly breezy conditions
developing, fire danger may be increasing across portions of the
county warning area. Stay tuned.

.LONG TERM...(Saturday through Wednesday)

Overview...Dry conditions will prevail over the weekend with
temperatures a few degrees cooler across the north and central on
Saturday.  Southwest flow aloft with embedded Pacific disturbances
will bring slight chances of afternoon and evening thunderstorms
across the area Monday through Wednesday with above normal
temperatures.

Discussion...Synoptic pattern on Saturday morning features upper low
moving across into southern Manitoba/Ontario with upper ridge
building into the northern Rockies.  A dry and stable airmass will
prevail across west and central Wyoming behind Friday`s cold frontal
passage.  On Sunday, surface high pressure over the northern plains
will bring a return SE flow up the Laramie range with moisture and
instability mainly confined to SE Wyoming; however, Casper and
possibly points further north along the I-25 corridor may see a few
afternoon and evening thunderstorms.

ECMWF is much more progressive than GFS bringing an upper low across
western and central Canada Sunday and Monday, with GFS holding most
of this energy across the NW U.S. through mid-week.  Both models
show weaker Pacific disturbances in W-SW flow moving across Wyoming
Monday through Wednesday with mean surface trough position
oscillating from eastern Wyoming into the northern High Plains.
Stronger, more organized thunderstorms will be along and east of
this trough axis with deeper moisture from SW monsoon remaining
south of the area.  Thus, expecting mostly isolated afternoon and
evening storms with gusty winds Monday through Wednesday with
possibly a few stronger storms in Johnson and Natrona Counties on
the edge of more favorable moisture and wind shear.  H7 temperatures
will average between 15c-18c Sunday through Wednesday, with central
basin highs expected to be mostly in the lower to mid 90s, 80s in
the western valleys.

&&

.AVIATION...12Z Issuance

VFR conditions will continue through the forecast period. Have made
some minor changes to afternoon winds at most of the TAF sites and
due to expected monsoonal flow, have kept in and adjusted VCTS/VCSH
mentions for KBPI, KCPR, KLND, KPNA, KRIW and KRKS from after 20Z-
05Z time frame.

&&

.FIRE WEATHER...

Expect another warm to hot day today. Monsoonal moisture will spread
northward into the area and bring an increased chance of mainly
afternoon showers or thunderstorms, especially across the southern
half of the state. Relative humidity will still fall to around 15
percent in many of the lower elevations in northern and central
Wyoming. Wind should again remain light to moderate outside of any
shower or thunderstorm. Mixing and smoke dispersal will be range
from fair to good in the basins to very good to excellent in the
normally breezier areas.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Troutman
FIRE WEATHER...Hattings



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